Field
The subject matter of the present disclosure generally relates to etch processes and materials for increasing the etch rate of glass wet etch processes without increasing the temperature of the etchant, and more specifically to increasing the etch rate of glass wet etch processes without increasing the temperature or the hydrofluoric acid content of the etchant, particularly for aluminosilicate glasses.
Technical Background
Wet chemical etching of silicate glass in aqueous hydrofluoric (HF) acid solutions has been studied for many years. General industry practice usually involves the use of a binary mixture of HF and a strong secondary mineral acid, such as hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3), among others. The addition of these secondary acids generally enhances the rate of etching.
HF dissolved in water is a weak acid. Solutions of HF are known to contain H+, F−, HF2-ions and un-dissociated HF molecules. However, HF is the best performing, or one of the best performing chemicals, in terms of the ability to appreciably dissolve silica-containing materials such as glass. As a result, and despite the expense and effort necessary to successfully manage the significant and well-known environmental and health risks associated with its use, HF is widely utilized for many applications where silica and other like materials are to be cleaned or dissolved.
For a given glass composition, the basic factors that affect the rate of etching of glass include the concentration of HF acid, the etchant temperature, and the presence and amount of physical agitation (whether by flow, stirring, application of acoustic energy, or other means). Increasing HF acid concentration generally increases the etch rate at a given constant temperature. Similarly, if HF acid concentration is held constant, increasing the temperature will also increase the etch rate. In most industrial applications, a high etch rate is usually necessary to enable an acceptably high throughput to be achieved. This is usually accomplished by using an elevated process temperature (a heated etchant) and/or by using an etchant with relatively high HF concentration.
Etchants with higher HF concentration increase the corrosion rate of most metal alloys, namely, bolts, rivets and any other similar degradable components within etch systems and within associated vapor recovery systems. In some cases certain types of heaters and/or chillers, which have to be immersed in the etching solution to enhance their heat transfer effectiveness, also fall prey to the harsh acid concentration, resulting in the need for frequent system maintenance to ensure that equipment integrity is not compromised. Also, at increased HF concentrations, the rate of evaporation of HF increases. This requires the use of enhanced-performance vapor recovery and safety systems to prevent hazardous gasses from escaping.
Similar issues arise if etchant temperature is increased. In addition to increased rate of corrosion of degradable components of the system, the rate of evaporation of gases increases with increase in temperature, requiring enhanced vapor recovery and safety units and more frequent maintenance in order to ensure continuous and safe operation.
In accord with the foregoing, it would be desirable achieve an increase in glass etch rates without having to increase etchant temperature and/or HF concentration.